The current work is part of a study of the cell biology and cell physiology of human intestinal protoan pathogens. Previous reports were focussed predominantly upon E. histolytica. The present report describes studies with Giardia lamblia in the following areas: 1) The role of reducing agents in promoting attachment, growth and resistance to killing by oxygen. 2) The kinetics of and requirements for attachment in a model system and the effects of host factors and chemotherapeutic agents upon attachment. 3) Interactions of G. lamblia trophozoites with complement. 4) Permeability and surface properties of cysts. 5) Isolation of new strains. 6) Antibody response in serum and secretions.